Séminaire IPAG

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A Chapter from the Book of Planet Formation: Pebbles, Planetesimals, and Turbulence

jeudi 7 mai 2026 - 11h00

Hubert Klahr - MPIA
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The small bodies of the Solar System — asteroids, Trojans, comets, and Kuiper Belt objects — are surviving remnants of the earliest stages of planet formation. These planetesimals were the building blocks from which planets emerged 4.5 billion years ago, through mutual collisions and the accretion of millimetre- to centimetre-sized solids known as pebbles. Yet the fundamental question of where and when planetesimals formed within the solar nebula remains unresolved. In this talk, I will present recent models exploring how hydrodynamic instabilities of the thermal baroclinic type, such as the Vertical Shear Instability and Convective Overstability, together with particle-feedback-driven processes such as the Streaming Instability, generate turbulence in magnetically inactive regions of protoplanetary disks. This turbulence governs the evolution of solid particles: it regulates the growth of dust grains into pebbles, drives their vertical stirring, and promotes their concentration into dense pebble clouds. Under the right conditions, these clouds can become gravitationally unstable and collapse into planetesimals. Using numerical simulations with a Lagrangian hydrodynamics code, we find that the collapse of a single pebble cloud does not produce a single object, but rather a cluster of many planetesimals. The resulting size distribution is consistent with observations of asteroids, comets, and Kuiper Belt objects. In this way, the minor bodies of the Solar System provide a fossil record of the turbulent environments in which they were born — allowing us to read, in today’s small bodies, a chapter from the book of planet formation.

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Hôtes : Geoffroy Lesur

Salle Manuel Forestini, 414 rue de la piscine, 38400 Saint Martin d'Hères